Casual Water
Casual water is temporary water on the course that gives you relief without penalty — in the current rules it is called "temporary water".
Casual water is the older term for puddles and accumulations of water that form after rain or irrigation and do not belong to the course as designed. Since the 2019 rules reform the official designation is temporary water; in the vocabulary of many golfers casual water lives on. It counts as an abnormal course condition: if your ball lies in it or the water interferes with your stance or swing, you get relief without a penalty stroke. Important: the water must be visible before or after you take your stance. Mere dampness or mud is not enough.
In practice the relief works like any free drop. An example: after heavy rain your ball lies in a puddle in the middle of the fairway. You look for the nearest point where neither ball, stance nor swing would be touched by the water — not nearer the hole. From there you drop within one club length and play on, without penalty. The same rule applies in the rough and in the general area, but not for water in a penalty area, because separate rules apply there.
Typical mistake is dropping at the most comfortable spot instead of the nearest one. The reference point is the nearest point of complete relief, even if that takes you into the rough or behind a tree. Also wrong: taking relief because the ball is only lying on damp ground. Without visible water there is no relief. You can read the details of the procedure under the term free drop.
FAQ
When do I get relief without penalty for water on the course?
When visible water is present before or after you have taken your stance and it touches your ball, stance or swing. Mere dampness or mud is not enough. The rule for temporary water then applies and you may drop without a penalty stroke.
How exactly do I drop my ball out of a puddle on the fairway?
You look for the nearest point where neither ball, stance nor swing would touch the water — and which is not nearer the hole. From there you drop within one club length and play on. You can find the details of the procedure under the term free drop; what matters is: not the most comfortable spot counts, but the nearest one.
Does the relief also apply when my ball lies in the water in a penalty area?
No, that is a common mistake. Inside a penalty area the water hazard's own rules apply, even if there is only a rain puddle there. You either play the ball as it lies or take the penalty area relief with a penalty stroke.